A CPP-GMR head is considered as one promising sensor to replace the conventional CIP (current in plane) GMR head for over 200 Gb/in2 recording density. In a typical CPP-GMR sensor, a bottom synthetic spin valve film stack is employed for biasing reasons and a CoFe/NiFe composite free layer is conventionally used following the tradition of CIP-GMR technology. GMR spin valve stacks have a configuration in which two ferromagnetic layers are separated by a non-magnetic conductive layer (spacer). One type of CPP-GMR sensor is called a metallic CPP-GMR that can be represented by the following configuration which has a copper spacer between the inner pinned (AP1) layer and free layer: Seed/AFM/AP2/Ru/AP1/Cu/free layer/capping layer. One of the ferromagnetic layers is a pinned layer in which the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) or pinning layer. The pinned layer may have a synthetic anti-parallel (SyAP) structure wherein an outer AP2 layer is separated from an inner AP1 layer by a coupling layer such as Ru. The second ferromagnetic layer is a free layer in which the magnetization vector can rotate in response to external magnetic fields. The rotation of magnetization in the free layer relative to the fixed layer magnetization generates a resistance change that is detected as a voltage change when a sense current is passed through the structure. In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to the layers in the stack. Alternatively, in the CIP sensor, the sense current passes through the sensor in a direction parallel to the planes of the layers in the sensor stack.
Ultra-high density (over 100 Gb/in2) recording requires a highly sensitive read head. To meet this requirement, the CPP configuration is a stronger candidate than the CIP configuration which has been used in recent hard disk drives (HDDs). The CPP configuration is more desirable for ultra-high density applications because a stronger output signal is achieved as the sensor size decreases, and the magnetoresistive (MR) ratio is higher for a CPP configuration. An important characteristic of a GMR head is the MR ratio which is dR/R where dR is the change in resistance of the spin valve sensor and R is the resistance of the spin valve sensor before the change. A higher MR ratio is desired for improved sensitivity in the device and this result is achieved when electrons in the sense current spend more time within the magnetically active layers of the sensor. Interfacial scattering which is the specular reflection of electrons at the interfaces between layers in the sensor stack can improve the MR ratio and increase sensitivity.
Another type of sensor is a so-called confining current path (CCP) CPP GMR sensor where the current through the Cu spacer is limited by the means of segregating metal path and oxide formation. With a current confining (CCP) scheme, CPP GMR performance can be further improved. An example of a CCP CPP-GMR sensor has the following configuration: Seed/AFM/AP2/Ru/AP1/Cu/CCP layer/Cu/free layer/capping layer where the CCP layer is sandwiched between two copper layers.
In a CPP operation mode, a tunnel magnetoresistive (TMR) head is another candidate for realizing high sensitivity and high recording density. In this design, the non-magnetic conductive layer between the pinned layer and free layer in the GMR stack is replaced by an insulating layer such as AlOx or MgO. When the magnetoresistive element is a magnetic tunnel junction (MTJ), the tunneling (insulating) layer may be thinned to give a very low RA (<5 ohms-μm2).
A MTJ element in a magnetic device such as a read head may be based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. The bottom layer in the MTJ element is typically comprised of a seed layer such as NiFeCr or a Ta/NiCr composite which promotes a (111) lattice orientation in overlying layers. Generally, an AFM layer, ferromagnetic “pinned” layer, tunnel barrier layer, ferromagnetic “free layer”, and a capping layer are successively formed on the seed layer to complete the MTJ stack. The pinned layer has a magnetic moment that is fixed in the “x” direction, for example, by exchange coupling with the adjacent AFM layer that is also magnetized in the “x” direction. The thin tunnel barrier layer above the pinned layer is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons.
The free layer has a magnetic moment that is either parallel or anti-parallel to the magnetic moment in the pinned layer. The magnetic moment of the free layer may change in response to external magnetic fields and it is the relative orientation of the magnetic moments between the free and pinned layers that determines the tunneling current and therefore the resistance of the tunneling junction. When a sense current is passed through the MTJ stack in a direction perpendicular to the layers therein, a lower resistance is detected when the magnetization directions of the free and pinned layers are in a parallel state (“1” memory state) and a higher resistance is noted when they are in an anti-parallel state or “0” memory state.
One indication of good device performance is a high magnetoresistive (TMR) ratio which is dR/R where R is the minimum resistance of the MTJ element and dR is the maximum change in resistance observed by changing the magnetic state of the free layer. In order to achieve desirable properties such as a specific junction resistance x area (RA) value, a high dR/R value, and a high breakdown voltage (Vb), it is necessary to have a smooth tunnel barrier layer that is promoted by a smooth and densely packed growth, such as a (111) texture for the AFM layer, pinned layer, and seed layer. Other desirable magnetic properties for an MTJ are a small interlayer coupling field (Hin) between the pinned layer and free layer, and a strong exchange coupling field (Hex) between the AFM layer and pinned layer is important to maintain the pinned layer magnetization in a certain direction.
For better head performance, it is always desirable to have a larger Hex to ensure pinning robustness between the AFM and AP2 layers and a smaller pinning dispersion. In other words, a larger Hex/Hc ratio is needed to suppress pinning field related noise. Improvement in the exchange bias properties may be achieved by proper selection of the seed layer, AFM layer, and pinned layer. However, further optimization than heretofore achieved is necessary for ultra-high density recording heads. Moreover, for ultra-high density read head applications, thinner full film stack thickness is preferred in order to achieve high resolution and high signal to noise ratio (SNR). Since reducing the magnetic layer thickness of a pinned layer or free layer, for example, will reduce the MR ratio, a better choice would be to decrease a non-magnetic layer thickness.
In U.S. Pat. No. 7,228,619, a seed layer comprising a magnetic material is deposited on an etched gap layer. The seed layer may be a bilayer with a lower NiFe layer and an upper Cu layer.
U.S. Pat. No. 7,218,487 describes a spin valve in which the bottom layer is a non-magnetic layer including at least one of Ta, Hf, Nb, Zr, Ti, Mo, and W and an overlying seed layer is comprised of a NiFe alloy or a NiFeY alloy where Y is at least one of Cr, Rh, Ta, Hf, Nb, Zr, and Ti.
In U.S. Patent Application Publication 2007/0169337, a CoFeON seed layer is formed between a write gap layer and a second pole pedestal. A small amount of oxygen in a CoFeN layer reduces coercivity and improves writer performance.